feat(notcore): add shared types, config, paths, error crate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/notcore/src/error.rs

@@ -0,0 +1,25 @@
+use std::path::PathBuf;
+
+#[derive(Debug, thiserror::Error)]
+pub enum NotfilesError {
+    #[error("config file error: {0}")]
+    Config(String),
+
+    #[error("package not found: {name}")]
+    PackageNotFound { name: String },
+
+    #[error("conflict at {path}: {reason}")]
+    Conflict { path: PathBuf, reason: String },
+
+    #[error("path error: {0}")]
+    Path(String),
+
+    #[error("state file error: {0}")]
+    State(String),
+
+    #[error("{0}")]
+    Io(#[from] std::io::Error),
+
+    #[error("{0}")]
+    Other(String),
+}